This invention relates to a magnetic recording medium for recording and reproducing at a high density, a glass substrate for the magnetic recording medium and a method of manufacturing the same.
Recently, a magnetic disk having a high recording density has been demanded. To achieve the high recording density, it is important to reduce a flying height of a magnetic head for a surface of a magnetic recording medium. In recent years, it has been required that the flying height falls within the range between 0.1 and 0.2 μm. To satisfy such a requirement, the magnetic disk having the high recording density has been developed as disclosed in Japanese Unexamined Patent Publications Nos. H5-303735 and H8-293177.
In the magnetic disk disclosed in the above-mentioned Japanese Unexamined Patent Publication No. H5-303735, an annular-shape substrate (glass substrate) has a surface roughness Ra of 1 μm or less and a flatness of 1 μm or less. Thereby, the flying height of the magnetic head for the magnetic disk is reduced to realize the high recording density. In the above-mentioned conventional technique, additional disclosure is made about a polishing method for smoothing the surface roughness of the annular-shaped substrate (glass substrate), and the glass substrate with its surface finished in a super-smoothing manner so that the surface roughness is set to 0.3 nm or less by Ra and 5 nm or less by Rmax.
Further, disclosure is made about a magnetic disk which is equipped in a dynamic head loading type magnetic disk drive in the Japanese Unexamined Patent Publication No. H8-293177. In the above-mentioned dynamic head loading type magnetic disk drive, the head is separated from the disk during a stop operation instead of the CCS drive in which the head halts on the medium during start/stop operations of the magnetic disk. This magnetic disk is structured by forming a magnetic layer having a super smooth finished magnetic surface and a protection layer having a super smooth finished protection surface on a super smooth finished substrate. Thereby, a magnetic distance between the head and the medium is reduced to realize the high recording density. Further, additional disclosure is made about a method for reducing a distance (effective flying height) between the surface of the magnetic layer and the magnetic head by reducing the film thickness of the protection layer.
In the above-mentioned conventional magnetic recording medium, the magnetic distance between the head and the medium can be sufficiently reduced by setting Rmax and Ra to predetermined values or less. However, according to inventor's research, it has been found out that the magnetic characteristic, such as the coercive force and the S/N ratio, and the record/reproduce characteristic do not sufficiently satisfy the recent strict demand in the conventional magnetic recording medium. Namely, even when the magnetic recording media are manufactured in the condition that the surface roughness of the glass substrate and the manufacturing condition of the magnetic layer formed thereon are made uniform, the magnetic disk having an excellent magnetic characteristic can not be always obtained. The inventors have enthusiastically researched about this cause. As a result, it has been concluded that the cause results in the variation of the surface roughness of the glass substrate, and the crystal growth of a thin film formed thereon, such as the magnetic layer, is disturbed due to the variation to deteriorate the magnetic characteristic and the record/reproduce characteristic. That is, the surface state (the surface roughness and the variation of the surface roughness) of the glass substrate relates to the crystal grain of the underlying layer and the magnetic layer which are formed thereon. Therefore, it is necessary to suppress the surface roughness of the glass substrate and equalize the crystal grains of the thin film formed thereon to obtain the magnetic recording medium which has an excellent magnetic characteristic and which is capable of recording and reproducing at the high density.
On the other hand, the relation between Rmax and Ra has not been considered at all in the above-mentioned conventional magnetic recording medium. Therefore, even if the substrate is excessively smoothed, the magnetic characteristic of each magnetic recording medium is fluctuated when a plurality of magnetic recording media are manufactured. Namely, even when the magnetic media are manufactured in the condition that the surface roughness of the glass substrate and the magnetic layer formed thereon are kept in the same manufacturing condition, the magnetic disk having stable and excellent magnetic characteristic can not be always obtained. In particular, the variation of the magnetic characteristic remarkably occurs when the magnetic disk is manufactured by the use of the known in-line type sputtering method.
Therefore, inventors has enthusiastically researched the causes. As a result, it has been found out that the relation (Rmax/Ra) between the surface roughness Rmax and Ra is not specified in the conventional techniques. Consequently, it has been concluded that the fluctuation of the magnetic characteristic of each magnetic disk may occur with the variation of the above-mentioned relation (Rmax/Ra).